The present invention relates generally to electronic controls for hydrostatic front wheel drives and more particularly to a three mode hydrostatic drive in which the hydrostatic drive wheels can be nondriving, nondriving until a predetermined main drive wheel slip is exceeded, or continuously driving.
In the past, hydrostatic drive wheels have been developed which automatically shifted between two or more torque levels in response to transmission ratio and/or hydraulic system pressure. These systems were continuously powered to provide supplementary hydrostatic drive for the main drive and had no provision for operating only on demand when the main drive loses traction and slips beyond a predetermined percentage. Similarly, previous systems have set the pressure of the hydrostatic drive at one or more levels rather than having the hydrostatic drive infinitely speed variable with a predetermined percentage of overspeed to provide a continuous, positive supplement to the main drive.